The present invention relates to a test system for identifying open leads in EEG testing. More particularly, the invention relates to a system for applying a distinctive test signal to each electrode of a multiple lead EEG system for detecting open circuits which may occur during a patient run.
EEG systems are widely used in the medical field, with the usual system including multiple leads that are secured to the scalp and/or other extremities of the patient to pick up electrical signals therefrom. In such testing it is possible for electrodes to become detached from the scalp during the test run. However, due to the low magnitude signals which are being recorded, the recorded output may appear to be normal even with such electrodes open. Thus, an undetected open circuit condition can lead to serious errors in interpretation of the patient record.
More specifically, when an electrode opens, the noise level of the amplifier increases due to the high source impedance presented to the amplifier with an open input. In addition, when the amplifiers are connected in multiple chains, it is possible for the voltage generated by the other electrodes in the system to divide among the amplifiers according to amplifier input impedance, whereby voltages from the other electrodes may appear at the input of the amplifier associated with the open electrode. Further, under some recording conditions, the open lead input may assume a ground or reference potential giving a monopolar recording rather than the differential input expected. Thus, a normally appearing record may be associated with an open electrode, due to either amplifier noise, ground referenced recording or voltage summing with other intact electrodes.
In conventional EEG usage, it is usual to perform an electrode impedance test on each electrode before an EEG run. This test, however, does not identify electrodes which become high or open during the EEG run.